Atomic Number (Z) Grade Shielding Materials and Methods of Making Atomic Number (Z) Grade Shielding

ABSTRACT

In some aspects, this disclosure relates to improved Z-grade materials, such as those used for shielding, systems incorporating such materials, and processes for making such Z-grade materials. In some examples, the Z-grade material includes a diffusion zone including mixed metallic alloy material with both a high atomic number material and a lower atomic number material. In certain examples, a process for making Z-grade material includes combining a high atomic number material and a low atomic number material, and bonding the high atomic number material and the low atomic number together using diffusion bonding. The processes may include vacuum pressing material at an elevated temperature, such as a temperature near a softening or melting point of the low atomic number material. In another aspect, systems such as a vault or an electronic enclosure are disclosed, where one or more surfaces of Z-grade material make up part or all of the vault/enclosure.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application claims the benefit of and priority to U.S.Provisional Patent Application No. 62/199,032, filed on Jul. 30, 2015,and titled “Additional Methods of Making Z-Grade,”; U.S. ProvisionalPatent Application No. 62/240,604, filed on Oct. 13, 2015, and titled“Additional Methods of Making Z-Grade,”; and U.S. Provisional PatentApplication No. 62/368,248, filed on Jul. 29, 2016, and titled“Additional Methods of Making Z-Grade,” where the contents of eachprovisional application are hereby incorporated by reference in theirentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention described herein was made in part by an employee of theUnited States Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefore.

TECHNICAL FIELD

In some aspects, this disclosure relates to improved Atomic Number (Z)grade (“Z-grade”) materials, such as those used for shielding, systemsincorporating such materials, such as Z-grade vaults, Z-grade electronicenclosures, and processes of making Z-grade materials.

BACKGROUND

Satellites and instruments, among other things, may require shielding orspot shielding when in orbit or in other environments where there isexposure to radiation. Thus, shielding may increase the lifetime ofionizing radiation sensitive components. Other applications may includepiping, housing, or suits designed to protect persons or materials fromradiation. In various applications, shielding may help provide hardwaredesign for increased orbital lifetimes, and enable out of low earthorbit (LEO) missions by using shielding for sensitive components. Forgeotransfer orbit (GTO), however, the radiation levels are around atleast 10 times the level of LEO. In Jovian environments, the amount ofradiation is still higher.

Z-shields made from Z-grade material may provide cost-effectiveshielding for such systems by utilizing sheets or pieces of metal withdifferent materials/densities, and thus including different atomicnumbers (Z). For example, a higher density metal and lower density metalmay be used together. The low atomic number materials slow high energyprotons and electrons via collision more effectively without theproduction of Bremmstrahlung radiation. At lower energies, high atomicnumber materials can also slow protons and electrons with reducedBremmstrahlung radiation. At the same time, the ability to use the highatomic number material may reduce the thickness of the overallshielding. As one example, known shielding applications utilize atypical outer skin of larger spacecraft with around 300 mils ofaluminum, and combines this with additional spot shielding using higheratomic number materials that takes advantage of inherent low atomicnumber shielding on the outside surface.

For smaller enclosures, known products may utilize a 50 mil thickaluminum skeleton or shell with limited shield potential. For anenclosure that must last for three to six months (as in many standardsmissions) or longer, this may be insufficient or may only last for theselimited periods.

While some Z-grade applications are known, additional compositions,applications, and synthesis methods may be desired.

SUMMARY

This Summary provides an introduction to some general concepts relatingto this disclosure in a simplified form, where the general concepts arefurther described below in the Detailed Description.

In some aspects, this disclosure relates to improved Z-grade materialsand synthesis methods. For example, in one aspect, a Z-grade alloymaterial is disclosed. In some examples, the Z-grade material includes ahigh atomic number material, a low atomic number material, where theatomic number of the low atomic number material is lower than the atomicnumber of the high atomic number material (in some examples, if alloysor other combinations are used for either material, any atomic numbersof the low atomic number material are lower than any atomic numbers ofthe higher atomic number material). The low atomic number material maybe bonded to the high atomic number material. The Z-grade material mayinclude a diffusion zone, the diffusion zone including a mixed metallicalloy material, the alloy material including both the high atomic numbermaterial and the lower atomic number material. The use of the disclosedZ-grade materials allows for a one-piece shielding that has sufficientmass thickness (areal density) while reducing the physical thickness(volume) of the shielding. This provides the ability to provideeffective radiation shielding in reduced volume or thicknessapplications such as, for example, small satellites, instrumentation,confined spaces and dimensions, etc.

In some examples, the diffusion zone of the Z-grade material is at least0.5 mil in thickness, in others it is at least 5 mil in thickness, andin others at least 10 mil in thickness. In certain examples, thethickness of the diffusion zone is equal to at least 10% of a thicknessof the thinner of the high atomic number material and the low atomicnumber material (or both, if they have equal or essentially equalthickness). In other examples, it is essentially equal to the thinner ofthe two. In still other examples, the diffusion zone is actually thickerthan the thinner of the two.

In some embodiments, the high atomic number material comprises one ormore of tantalum, tungsten, or a copper-tungsten alloy. In certainexamples, the low atomic number material comprises one or more ofaluminum or titanium. In some examples, the Z-grade material alsoincludes an aluminum layer bonded the Z-grade material (e.g. bonded tothe low atomic number material). In certain examples, the density of thediffusion zone is between or varies between around 4.4 g/cm³ and about16.7 g/cm³ along the gradient. In some examples, the diffusion zone is agraded metallic alloy.

In some examples, the Z-grade alloy material has an areal density of atleast about 3.0 g/cm². In various examples, the alloy material has anoverall thickness of the Z-grade alloy material of about 240 mils orless, about 190 mils or less, about 140 mils or less, or about 100 milsor less.

In accordance with another aspect, systems are disclosed. In someexamples, the system is a housing, a vault, shield, or an enclosure(such as an electronic enclosure). In some examples, the system is aZ-grade vault. The Z-grade vault may include one or more surfaces ofZ-grade material, where the one or more surfaces may include a highatomic number material and a low atomic number material, where theatomic number of the low atomic number material is lower than the atomicnumber of the high atomic number material, and where the low atomicnumber material is diffusion bonded to the high atomic number material.In some examples, the areal density of the Z-grade material is at leastabout 2.5 g/cm², and wherein an overall thickness of the Z-grade alloymaterial is about 240 mils or less, about 190 mils or less, about 140mils or less, or about 100 mils or less. In some examples, the arealdensity of the one or more surfaces of Z-grade material is at leastabout 3.0 g/cm². In various embodiments, the Z-grade material furthercomprises an aluminum layer bonded to the low atomic number material.

In accordance with another aspect, processes are disclosed. In someexamples, the process may include combining a high atomic numbermaterial and a low atomic number material, where the atomic number ofthe low atomic number material is lower than the atomic number of thehigh atomic number material, and bonding the high atomic number materialand the low atomic number together using diffusion bonding to form aZ-grade material. In various examples, the diffusion bonding includesvacuum pressing the high atomic number material and the lower atomicnumber material at an elevated temperature.

In some examples, the method further includes (in addition to thediffusion bonding) vacuum pressing the Z-grade material at an elevatedtemperature. In certain examples, the elevated temperature is near asoftening or melting point of the low atomic number material. In variousembodiments, the process also includes cooling the Z-grade materialunder vacuum.

In some embodiments, the diffusion bonding includes plasma spraying thelow atomic number material onto a sheet of the higher atomic numbermaterial. In certain examples, the diffusion bonding includes weldingthe low atomic number material onto a sheet of the higher atomic numbermaterial using an electronic beam gun. In various embodiments, thediffusion bonding includes heating the low atomic number material underan inert atmosphere or a vacuum to its melting temperature, and coatinga sheet (or other piece) of the high atomic number material with themelted low atomic number material. In some examples, the diffusionbonding includes ultrasonic layering of the low atomic number materialonto the high atomic number material.

In various examples of the process, the high atomic number materialincludes one or more of tantalum, tungsten, or a copper-tungsten alloy,and the low atomic number material comprises one or more of aluminum,titanium and vanadium. In certain examples, the formed Z-grade materialincludes a diffusion zone, where the diffusion zone includes a mixedmetallic alloy material, the alloy material including both the highatomic number material and the lower atomic number material.

These summary descriptions are merely provide examples of the processesand/or process steps that may be performed in one or more embodiments.In certain embodiments, materials and methods include additionalcombinations or substitutions. To that end, other details and featureswill be described in the sections that follow. Any of the featuresdiscussed in the embodiments of one aspect may be features ofembodiments of any other aspect discussed herein. Moreover, additionaland alternative suitable variations, features, aspects and steps will berecognized by those skilled in the art given the benefit of thisdisclosure.

These and other features, advantages, and objects of the presentinvention will be further understood and appreciated by those skilled inthe art by reference to the following specification, claims, andappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the disclosure will now be described by way ofexample only and with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of an example Z-grade material.

FIG. 2 is a graph illustrating the stopping power of Z-grade withvarying materials, and the material densities.

FIGS. 3-5 are EDAX images of titanium, vanadium, and tantalum in aZ-grade material.

FIG. 6 is a back scattered SEM image of diffusion bond between Ti andTa.

FIG. 7 is an EDAX image and data for the material shown in FIG. 6.

FIG. 8 illustrates the microhardness/Knoop hardness test (KHN) forvarious layers (Ti, diffusion zone, and Ta) of a Z-grade material aftervarious experimental conditions.

FIG. 9 illustrates the microhardness/Knoop hardness test (KHN) adiffusion zone.

FIGS. 10A and 10B illustrate the microhardness/Knoop hardness test (KHN)for diffusion zones after various experimental conditions.

FIGS. 11A-11C are scanning electronic microscope images of Ti—Tadiffusion zones after various experimental conditions.

FIGS. 12A-12C are spectra of various areas of a Ti/Ta Z-grade material.

FIGS. 13A and 13B show graphs of ionizing doses as a function of arealdensity for Al, Ta, and Al/Ta samples.

FIG. 14 is an electron image of an example Z-grade material.

DETAILED DESCRIPTION

The examples, materials and methods of described herein provide, interalia, Z-grade materials, shielding components or systems, and processesof making the same. These and other aspects, features and advantages ofthe disclosure or of certain embodiments of the disclosure will befurther understood by those skilled in the art from the followingdescription of example embodiments. In the following description ofvarious examples, reference is made to the accompanying drawings, whichform a part hereof. It is to be understood that other modifications maybe made from the specifically described methods and systems withoutdeparting from the scope of the present disclosure.

It is also to be understood that the specific material, systems, devicesand processes illustrated in the attached drawings, and/or described inthe following specification, are simply exemplary embodiments. Hence,specific dimensions and other physical characteristics relating to theembodiments disclosed herein are not to be considered as limiting.Moreover, the figures of this disclosure may represent the scale and/ordimensions according to one or more embodiments, and as such contributeto the teaching of such dimensional scaling. However, the disclosureherein is not limited to the scales, dimensions, proportions, and/ororientations shown in the figures.

In some aspects, this disclosure relates to improved Z-grade materialssuch as a Z-grade alloy material. FIG. 1 shows an example schematic viewof a Z-grade material including a high atomic number material 101, a lowatomic number material 102, where the atomic number of the low atomicnumber material is lower than the atomic number of the high atomicnumber material (in some examples, if alloys or other combinations areused for either material, any atomic numbers of the low atomic numbermaterial are lower than any atomic numbers of the higher atomic numbermaterial). The low atomic number material may be bonded to the highatomic number material. The Z-grade material in this example alsoincludes a diffusion zone 103, the diffusion zone including a mixedmetallic alloy material, the alloy material including both the highatomic number material and the lower atomic number material. In someexamples, the diffusion zone includes a gradient of materials. Forexample, the example schematic of FIG. 1 shows a region with arelatively higher concentration of the high atomic number material 104,and a region with a relatively higher concentration of the low atomicnumber material 105. In some examples, the composition of the diffusionzone may be relatively uniform, however.

In some examples, the diffusion zone of the Z-grade material is at least0.5 mil in thickness, at least 1 mil, at least 2.5 mil, at least 4 mil,at least 5 mil, or at least 7.5 mil, while in others it is at least 10mil in thickness, 25 mil, at least 40 mil, and in others at least 50 milor 75 mil thickness. In some examples, it is 1-2.5, 1-5, 1-10, 5-10,10-50 or 10-20 mil in thickness. In certain examples, the thickness ofthe diffusion zone equal to at least 10% of a thickness of the thinnerof the high atomic number material and the low atomic number material(or both, if they have equal or essentially equal thickness). In otherexamples, it is essentially equal to the thinner of the two. In stillother examples, the diffusion zone is actually thicker than the thinnerof the two. As one example, a titanium layer may be approximately 100mil, a tantalum layer approximately 8-10 mil, and the diffusion zonebetween the two is approximately 10 mil. In some examples, any of thelayers may be between 1-500 mil, such as 1-100, 10-100, 1-10, 1-5, 5-10,1-25, 5-25, or 50-100 mil.

The Z-grade material may have an overall thickness suitable for itsparticular application, expected radiation levels, and/or applicablevolume constraints. In some examples, the overall Z-grade material is atleast 100 mil thick, in others at least 200, and in others at least 300.In other examples, the material provides the desired shieldingcharacteristics while remaining under 400 mil, or under 300 mil, under250 mil, under 200 mil, or under 150 mil. In comparison an aluminum (Al)shielding at 3.0 g/cm² has a thickness of 434 mils and a titanium (Ti6-4) shielding at 3.0 g/cm² has a thickness of 267 mils.

In some embodiments, the high atomic number material comprises one ormore of tantalum, tungsten, or a copper-tungsten alloy. In certainexamples, the low atomic number material comprises one or more ofaluminum, titanium and vanadium. In various examples, aluminum and/ortitanium materials are used to form an alloy with tantalum. Suitableexample materials include Al 6061 or Ti6-4. In some examples, theZ-grade material also includes an aluminum layer bonded the Z-gradematerial (e.g. bonded to the low atomic number material, such as beingbonded to titanium after a titanium/tantalum Z-grade material isformed). The optional addition of aluminum by diffusion bonding withtitanium or brazing enables another lower atomic number material to beadded. This may be advantageous for fast electron shielding where theBremstrahlung critical energy can be increased, so as to reduceBremstrahlung formation, when comparing aluminum to titanium. Titaniummay also be used as an adhesive interlayer between aluminum and tantalum(or other high/low atomic number materials).

In certain examples, the density of the diffusion zone is between orvaries between around 4.4 g/cm³ and about 16.7 g/cm³ along the gradient.In some examples, the diffusion zone is a graded metallic alloy. In someexamples, the density is at least 4.0 g/cm³ and up, 6.0 g/cm³ and up,8.0 g/cm³ and up, 10.0 g/cm³ and up, or 12.0 g/cm³ and up. In variousexamples, the areal density of the entire Z-grade material is between1.5 and 2.25 g/cm² or between 2.5 and 3.0 g/cm², or between 2.9 and 3.1g/cm². In some examples, it is 2.0 g/cm², 2.25 g/cm², 2.5 g/cm², 2.75g/cm², 2.9 g/cm², 3.0 g/cm², or 3.1 g/cm² or above, while in others itis between 1.6 and 1.7 cm², and other between 1.5 and 2.0 g/cm².

The Z-grade materials may provide an integrated, single piece ofshielding as opposed to prior systems using, e.g., additional spotshield of the second high atomic number material. This may beparticularly useful for small satellites, or small instruments housingapplications where shielding is needed in one or more areas, or toenclose an entire device/satellite/instrument/etc., but where there arevolume constraints affecting the amount and type of shielding that maybe utilized. By reducing shielding thickness through use of a highatomic number material (and optionally, as described herein, by makingthe low atomic number material denser via elevated temperatureprocesses) the Z-grade materials may help enable shielding of smallstructures with less volume impact, when compared to typical aluminumshielding. Indeed, for one application example, the significantreduction in spacecraft volume will benefit from one piece Z-gradeshields, as larger thickness aluminum shields would prohibit theincorporation of standard electronic cards in an enclosure, due tovolume constraints.

The Z-grade materials may have high densities for each material layer(as compared to layers obtained through prior formation methods), beflat and thus easy to incorporate into various systems, and have astrong weld or interface between layers.

In some examples, a Z-grade material (such as one with Al/Ta or Ti/Ta,i.e. an aluminum/tantalum or titanium/tantalum material) a can reducethe overall thickness in half compared to standard (e.g. aluminum only)shielding but providing same areal density. As illustrated in FIG. 13,the electron shielding of an example Z-grade material is modeled having30% greater shielding effectiveness and about the same proton shieldingeffectiveness compared to an aluminum shield. Specifically, the Z-shieldproperties have been estimated, using The Space Environment InformationSystem (SPENVIS) radiation shielding computational modeling, to have˜30% increased shielding effectiveness of electrons, at half thethickness of a corresponding single layer of aluminum.

The diffusion zone may enable a shielding property between that of ahigh Z material and a low Z material, without having to add anothermaterial layer. This can not only provide additional shielding benefits,but can help lower thickness and volume.

Example Material Characteristics

The remaining Figures of the application illustrate example propertiesthat the Z-grade material and/or its constituent materials may provide.

FIG. 2 is a graph illustrating the stopping power of Z-grade by modelingthe energy loss of a 49.3 MeV proton beam, with varying materials havingvarying densities and thicknesses, and illustrating how the Z-gradematerials may provide essentially equivalent shielding in conjunctionwith thinner materials.

FIGS. 3-5 are EDAX images showing diffusion of titanium and vanadiuminto tantalum, and illustration a very uniform gradient of the Ti and Tain the bonding area interface.

FIG. 6 is a back scattered SEM image of diffusion bond formed over 256hours between Ti and Ta.

FIG. 7 is a EDAX image and data illustrating a diffusion zone of 124microns and a higher CPS intensity for the titanium material for thematerial shown in FIG. 6.

FIG. 8 illustrates the microhardness/Knoop hardness test (KHN) for avarious layers (Ti, diffusion zone, and Ta) for 4, 16, and 256 hourdiffused Ta—Ti interface, where the diffusion zone was about 130microns.

FIG. 9 illustrates the microhardness/Knoop hardness test (KHN) for a 256hour diffused Ta—Ti interface, where the diffusion zone was about 130microns.

FIGS. 10A and 10B illustrate the microhardness/Knoop hardness test (KHN)for a 4 hour diffused Ta—Ti interface, where the diffusion zone wasabout 15 microns (FIG. 10A) and a 16 hour diffused Ta—Ti interface,where the diffusion zone was about 30 microns (FIG. 10B).

FIGS. 11A-11C are scanning electronic microscope images of a Ti—Tadiffusion zone after 4 hours at 890° C. (FIG. 11A), 16 hours (FIG. 11B)and 256 hours (FIG. 11C) and with 50 mPa of pressure (which is anexample possible value but not required). The lightest top layer is Ta,which most dramatically shows diamond scoring marks resulting fromadditional hardness tests, as it is a softer material. The diffusionzone is in the intermediate medium grey, which clearly increases in sizeas time increases, and the bottom, dark grey layers in these examples isTi6-4, which is a harder material and therefore only has smaller scoringmarks.

FIGS. 12A-12C are spectra of various areas of a Ti/Ta Z-grade material.FIG. 12A is the Ti layer (Ti6-4) which has around 88.5% Ti, around 5.9%Al, and around 2.4% V (in other examples the V content may be around4%). FIG. 12B shoes the spectra for the diffusion zone, where the amountof Ti is around 59.2% and the amount of Ta is about 28%, illustratingthat Ti may diffuse more readily compared to Ta. FIG. 12C shows thespectra for the Ta layers, which was around 90.1% Ta.

FIG. 13(a.) shows that with expected ionizing dose of 10-400 MeVprotons, the Al/Ta has similar shielding performance to Al atapproximately half the thickness. In FIG. 13(b.), a expected ionizingdose of 4-6.5 MeV electrons shows greater than 30% improvement inshielding effectiveness for Al/Ta over Al. The predominant radiationdose received behind the shielding samples originated from the protonionizing dose. In FIG. 13(a.), the dose levels appear below 1 kRad forAl and Al/Ta at areal densities above 1.7 g/cm², whereas Ta appearshigher. In FIG. 13(b.), the electron radiation dose at areal densitiesabove 1.7 g/cm² appear below 200 Rad for Al/Ta and Ta. At arealdensities greater than 2 g/cm², the electron ionizing dose for the Al/Taappears to be reduced almost completely. Overall, the expectedaccumulated total ionizing dose behind 3 g/cm² shielding will originatefrom proton radiation.

FIG. 14 is an electron image of an example Ti64/Ta material, with the Timaterial at the top of the image, a diffusion zone in the intermediatepart of the image, and the Ta material at the bottom of the image.

Making the Z-Grade Material

In accordance with another aspect, processes are disclosed. These mayutilize metallurgy techniques to make one piece radiation shielding withlayers of differing atomic numbers. In some examples, the process mayinclude combining a high atomic number material and a low atomic numbermaterial, where the atomic number of the low atomic number material islower than the atomic number of the high atomic number material, andbonding the high atomic number material and the low atomic numbertogether using diffusion bonding to form a Z-grade material. In variousexamples, the diffusion bonding includes vacuum pressing the high atomicnumber material and the lower atomic number material at an elevatedtemperature.

In some examples, the method further includes (in addition to thediffusion bonding) vacuum pressing the Z-grade material at an elevatedtemperature. In certain examples, the elevated temperature is near asoftening or melting point of the low atomic number material. In variousembodiments, the process also includes cooling the Z-grade materialunder vacuum.

For some specific examples, vacuum pressure diffusion bonding is used,where layers of titanium (Ti) and tantalum (Ta) are bonded at elevatedtemperatures underneath the melt temperature of the Ti6-4 titaniummaterial. As another example, Aluminum (Al) and a previously formedTi/Ta material is vacuum pressured diffusion bonded to make Al/Ti/TaZ-grade. This additional bonding uses welding at a relatively lowertemperature under vacuum, under the melt temperature of aluminum. Theseexamples utilize layering and diffusion bonding.

For another specific example, a diffusion gradient Z-grade may be formedwith titanium/tantalum. A Ti6-4 Ta sample may be produced via diffusionbonding at elevated temperature for extended time periods to allow alarge diffusion zone between titanium and tantalum to take place. Thisimproves the gradient of the Z-grade by having a greater distribution oftitanium and tantalum in the diffusion zone (again, different suitablematerials and metal or metallic alloys may be used in this and all otherexamples specifically identifying a material, and the elevatedtemperatures may be adjusted accordingly relative to the different melttemperature). This maximizes the Z-grade of the low atomic number tohigh atomic number by creating an extended diffusion gradient of thealloys between majority titanium to majority tantalum to form a gradeddiffusion zone alloy. In some examples, the density gradually increasesbetween the two starting materials.

The diffusion zone can thus be expanded, for example to 5 mils, which is10 times thicker than typical bonding applications. This additionaldiffusion zone may create a new shielding layer with an actual atomicnumber gradient. This can be further exploited to create intermediatedensities of graded alloys between a low atomic number and high atomicnumber material. In turn, this will shield fast electrons and heavyions, such that the radiation can be even further reduced with a simplermaterial lay-up while retaining the benefit of thickness reductions. Theaddition of aluminum on Ti/Ta has substantially improved the atomicnumber Z-grade with the addition of low atomic number aluminum.

The use of an additional aluminum layer (or simply Z-grade utilizingaluminum) reduces Bremstrahlung radiation in shielding applications forfast electron applications for the Jovian environment where the energycritical point for Bremstrahlung formation, i.e.E_(collision loss)=E_(radiative loss), is estimated at 51.0 MeV.Therefore the dominant 30 MeV Jovian electrons can be slowed down withreduced Bremstrahlung formation. The critical energy for titanium isestimated at 34.5 MeV. These critical energy calculations are from pg.41, W. R. Leo, “Techniques for Nuclear and Particle Physics Experiments,2nd Edition, Spring Verlag, Berlin, 1194, page 378 (which is expresslyincorporated by reference). In short, there is a significant reductionin Bremstrahlung formation with the incorporation of Al to Ti/Ta forfast electron shielding applications.

Any important feature of this disclosure is that for Ti/Ta Z-gradematerials (and others) these can be manufactured much more simply byusing high temperature vacuum press diffusion bonding. In this casethere are also radiation shielding benefits available by being able toshape the material using diffusion bonding techniques through use of,e.g. Ti and Ta. For example, one may add separate faces of a cube and/orframe pieces together (or other shapes/components) and under pressure(e.g. by screws) form a shaped structure such as an electronic enclosureto create a seamless joint, as a result of the diffusion bonding, asself-welding that can occur at the joints. For high energy particleradiation shielding, the low atomic number material can be used toshield fast electrons such as the 30 MeV electrons found in the Jovianenvironment or the earth electron belt. It can also shield high energyprotons, such as ones found from solar radiation or the earth innerproton belt with reduced Bremstrahlung formation. In many vaultapplications or enclosure applications 3 g/cm² aluminum has been used inthe past, where this past example is often about 434 mil thick.

A Z-grade with Al/Ta or Ti/Ta can reduce the thickness in half (or evenmore, as described below, e.g. by around 70% or 80%) with the same arealdensity. The electron shielding of the Z-grade with 50% thicknessreduction is modeled having 30% greater shielding effectiveness andabout the same proton shielding effectiveness. Recent shielding modeling(FIG. 2) show that Al/Ti/Ta and Ti/Ta Z-grades had slightly lowerstopping powers Al/Ti/Ta (10.1 MeV cm²/g) and Ti/Ta (9.4 MeV cm²/g)compared to baselines Al (13.8 MeV cm²/g) and Ti (11.5 MeV cm²/g). Theproton shielding models show that Ti/Ta and Al/Ti/Ta could be used as asubstitute for Al shielding applications. The Ti may also add astructural component.

Another advantage of the process is the vacuum pressing at elevatedtemperature is a relatively cheap technique. The other weldingtechniques such as ultrasonic and friction stir welding and electronbeam freeform fabrication can make the shielding materials additivelyusing powder, wire, foil, and sheet. These other methods take advantageof welding, diffusion bonding methods. The additive approaches alsoenable making 3D and multilayer constructions.

As mentioned above, another unique feature of this disclosure is theability to make an extended diffusion zone, which may create a gradedalloy between two materials. For example, extending the diffusion timesin high vacuum oven at an elevated temperature (e.g. 890° C., but otherappropriate temperatures based on the material may be used) may createan extended diffusion zone. These times may be 2 hours or hour, 4 hoursor more, 8 hours or more, 12 hours or more, 16 hours or more, 1 day ormore, two days or more, 4 days or more, 7 days or more, or 10 days ormore. In some examples, 4 hours of diffusion time at 890° C. resulted ina 17 micron diffusion zone. Raising the temperature (such as to 1200 or1300° C.), can further enhance the diffusion zone extension. Forexample, raising the temperature from 890° C. to 1200° C. will increasethe diffusion rate for the thermal dependence on the Arrhenius equation.It will also increase because titanium will be in the beta phase, a BCCstructure. In cases, the diffusivities differ between BCC and HCPbetween 1 and 2 orders of magnitude.

In some examples, a limiting factor in increasing the area of thediffusion zone is the diffusion coefficient of the tantalum and Ti-6-4in the alpha phase, hexagonal close pack phase, at 890° C. The diffusionrates of a hexagonal close packed phase can be up to 5 times slower thanin a body centered cubic phase. Tantalum is in a body centered cubicphase. Therefore increasing the temperature for the diffusion bondingmay increase the inter-diffusion between tantalum and titanium andincrease the size of the diffusion zone. If the temperature is raised to1200-1300° C., for example, the diffusion rate will increase. Othersuitable temperatures are around 900, 950, 1000, 1100° C. (or above).The only trade-off is that the titanium will have to go through a phasetransition and on cooling the tantalum/titanium layered material maycurl or warp slightly. But when this occurs, the material may beflattened, for example at a slightly elevated temperature, asillustrated below. Increasing the temperature and diffusing in the betaphase of titanium will also contribute to conditions for making a largerZ-grade in a shorter amount of time.

By providing an extended diffusion zone, this results in a new shieldingapproach where the Z-grade occurs with an actual density gradient.Current Z-grade systems are at best systems using multiple and separatelayers: such as spacecraft skin, avionics case, and a single layer ortwo layer spot shield.

For example, in formation of a Ti/Ta Z-grade material using extendeddiffusion times of 256 hours, this makes a new intermediate densitymaterial between Ti6-4 density of about 4.43 g/cm³ and Ta density ofabout 16.69 g/cm³. This enables a shielding property between that of ahigh Z material and a low Z material, without having to add anothermaterial layer. And the addition of aluminum by diffusion bonding withTi or brazing enables a yet another lower atomic number material to beadded. This is significant for fast electron shielding where theBremstrahlung critical energy can be increased, so as to reduceBremstrahlung formation, when comparing Al to Ti.

In these examples, the titanium can also be used as an adhesiveinterlayer between Al and Ta. This has been demonstrated with anAl/Ti/Ta material sample as described herein. First, a Ti/Ta material isdiffusion bonded together using Ti and Ta sheets at elevatedtemperatures at 850-890° C. (as an example). Then Al can be diffusionbonded to the Ti sheet of Ti/Ta in order to take advantage of Al beingable to bond with Ti at a lower temperature, underneath the meltingpoint of Ti. Therefore, low atomic number materials such as Al or alloysof Al can be adhesively bonded with Ti or similar reactive materials ofhigher melting point with tantalum or other refractory materials such astungsten or tungsten alloys, such as tungsten copper. The benefits ofthese Z-grade shielding arc for applications with small satellites orinstrument enclosures where volume reduction is important and the needto effectively shielding high energy particles necessitates a layeredapproach for volume and shielding effectiveness.

In some embodiments, the diffusion bonding includes thermal spaying,such as plasma spraying, the low atomic number material onto a sheet ofthe higher atomic number material. The thermal spraying may includeplasma spraying or RF plasma spraying. RF plasma spraying titanium oraluminum onto a tantalum sheet has a high chance of welding at theinterface. Both titanium and aluminum are known to alloy with tantalumonce the aluminum or titanium is added to the tantalum. The titanium oraluminum layered tantalum can then be vacuum hot pressed near thesoftening points of the low atomic numbered materials, e.g. the aluminumor titanium, to increase the density of the low atomic number materialon top of the tantalum sheet. Al 6061 or Ti6-4 plasma spray powder maybe used to take advantage of its alloy property.

In certain examples, the diffusion bonding includes welding the lowatomic number material onto a sheet of the higher atomic number materialusing an electronic beam gun. This may utilize a wide feed (e.g. a dualwire feed) and the electron beam gun to depositing the material. Forexample, the process may use Electron Beam Freeform Fabrication (EBF3)aluminum or titanium wire layered onto tantalum material (e.g. a sheet).This process would take advantage of the welding technology of the EBF3.The EBF3 method allows another way of adding a dense layer of low atomicnumber material or alloy onto the higher atomic number material, such astantalum or tungsten, which are refractory metals and hard to melt.After the process is done, the layered sheet material may have warped inshape due to the thermal stresses of the welded EBF3 material. Thisbilayer material can then be added to a vacuum hot press just below themelt temperature of the aluminum or titanium to soften and cool in thevacuum press to remove the warp. Again, Al 6061 or Ti6-4 may be used totake advantage of its alloy property.

In various embodiments, the diffusion bonding includes heating the lowatomic number material under an inert atmosphere or a vacuum to itsmelting temperature, and coating a sheet of the high atomic numbermaterial with the melted low atomic number material. As an example, thebonding may include casting Al or Ti on top of tantalum or tungsten orCuW sheet. This process may be by accomplished by heating Al or Ti oralloys thereof under an inert atmosphere or in vacuum to the meltingtemperature. The molten Al or Ti would then coat over the tantalum ortungsten material (e.g. a sheet). Al and Ti can form alloys withtantalum and thus make a strong interface with these materials (e.g. asheet of the material). On cooling, if the tantalum or tungsten sheetwarps due to thermal stress, then the resulting Al or Ti coated tantalumor tungsten sheet can be placed into a vacuum hot press and heated untilAl or Ti softens, just below the melt temperature, such that thepressure straightens the bilayer sheet. Then it is cooled while underpressure to retain the flatness.

In some examples, the diffusion bonding includes ultrasonic layering ofthe low atomic number material onto the high atomic number material. Forexample, the bonding may include ultrasonic layering of (a) Aluminum orTitanium foil onto tantalum or tungsten or CuW sheet, (b) tantalum ortungsten or CuW foil onto aluminum or titanium sheet, (c) tantalum ortungsten foil onto aluminum or titanium foil, or (d) aluminum ortitanium foil onto tantalum or tungsten foil. A forged or high densitylayer may be formed. In some examples, a larger area is made, e.g. atleast 10 v. 10 cm, or having at least 100 cubic centimeters in area (butlarger or smaller squares, rectangles, or other geometric or nonegeometric shapes are suitable for the Z-grade materials (made via thisexample process or others), such areas as 200 cubic centimeters or more,500 or more, 1000 or more, and so on). In some examples, the layers areflat, so the layered materials may be placed in a vacuum hot press tojust below the melt temperature of the low atomic number material, suchas aluminum or titanium. This may provide a strong interface (goodweld), a flat large area sheet, and high densities for each elementalmaterial or alloy. In this manner, however the initial Z-grade materialis made (e.g. ultrasonic v. plasma spraying or others), additionaldesirable characteristics may be obtained.

In various examples of the process, the high atomic number materialincludes one or more of tantalum, tungsten, or a copper-tungsten alloy,and the low atomic number material comprises one or more of aluminum ortitanium. In certain examples, the formed Z-grade material includes adiffusion zone, where the diffusion zone includes a mixed metallic alloymaterial, the alloy material including both the high atomic numbermaterial and the lower atomic number material.

Example Applications and Systems

The applications of the Z-grade material are numerous, but theseimproved materials may be particular advantages for satellites or otherspace applications such as shuttles. For example, a research payloadcould be made with the Z-graded radiation shields of varyingthicknesses. As another example, an engineered Z-grade radiationshielding vault may protect a system's electronic boards. Otherhousings, encloses, surfaces, or spot shields may be made. In someexamples, one or more surfaces and/or other pieces of Z-grade materialmay be fastened, attached, bonded or joined to each other, a framepieces or entire frame, or another object to form a partial or fullenclosure. In other examples, there may be a skin or surface on theexterior of a satellite or shuttle comprising the Z-grade material, or ahousing enclosing one or more parts or components, that comprise theZ-grade material in the entire housing or in one or more sections of thehousing. The improves processes described herein enable cost effectiveshielding for small satellite systems, with significant volumeconstraints, while increasing the operational lifetime of ionizingradiation sensitive components. This in turn may provide for increasedmission lifetimes, and enable, for example, out of low earth orbit (LEO)missions.

For example, the Al/Ta Z-grade material may offer a thickness reductionapproaching half of a typical 3 g/cm² (1.1 cm) Al shield. With materialsdimensions of approximately 10 cm×10 cm×10 cm (1000 cm³) the loss ofelectronics card volume area and cable volume would be 295 cm³ or −30%of the volume. A shielding thickness of a 0.5 cm Z-grade would only havea volume reduction of −14%. At the same time, the Z-grade materialperforms similar to Al for the proton environment and over 30% moreeffective at areal densities of 1.7 to 2.2 g/cm² for an electronenvironment. The addition of Z-grade shielding thus can offer thereduction of total ionizing dose on sensitive electronic components,such as memory cards and CMOS devices. The near complete elimination ofelectron radiation at areal densities greater than 2 g/cm² reduces thechance of internal charging effects on electronic that causes anomalies.The use of the Z-grade radiation shielding enables shieldingapplications in volume constrained small satellites and instrumentenclosures, where typical aluminum shielding is volume prohibitive.

As another example, a Ti/Ta Z-grade material may offer a thicknessreduction compared to known shielding materials (e.g. the standard 434mil Al shield) of up to about 70% of a typical 3 g/cm² (1.1 cm) Alshield, or even about 80%. In some embodiments, a Ti/Ta material has anoverall thickness of about 140 mils (i.e. about 0.36 cm) and an arealdensity of 3.0 c/cm², where the Ti layer is about 105 mils (i.e. about0.27 cm) and the Ta layer is about 40 mils (i.e. about 0.09 cm). For anexample incorporating the additional Al layer, a Z-grade material with(at least) the desired areal density has an Al layers of 0.23 cm, a Tilayers of 0.16 cm and a Ta layer of 0.09 (i.e. about 90 mils, about 63mils, and about 40 mils).

In some examples, a relatively thin Ti (or other low atomic numbermateriel) layer is used, diffusion bonded to a high atomic numbermaterial, and then an additional Al layer is brazed on (e.g. after thematerials are cleaned). The diffusion zone may relatively extended tomake the e.g. Ti layer even thinner by extending the gradient andlengthening the diffusion zone incorporating the high atomic numbermaterial. As another example, the diffusion conditions may be such thatthe large amounts (e.g. 50% or more, 70% or more, or 90% or more) oreven essentially the entire high atomic number material diffuses intothe lower atomic number material, providing increased density whilelowering overall thickness.

For these relatively thin materials (e.g. having a total thickness ofabout 190 mils or less, 160 mils or less, 150 mils or less, 140 mils orless, 125 mils or less, 110 mils or less, 100 mils or less, 95 mils orless or 90 mils or less), that still provide an areal density sufficientfor shielding (e.g. around 3.0 or at least 3.0) one of more sheets ofthe materials may be connected to form a vault or housing (e.g. todefine a square or rectangular area, or any other shape as needed for aparticular electric component or other object that requires shielding).Where high degrees of thinness is needed, the amount of Tantalum (orother high atomic number material) may be increased and formed into analloy with a low atomic number material (e.g. Titanium) to form a thinbut dense layer. While this may not be as strong structurally as otherexamples, another layer of e.g. Ti (or another low atomic numbermaterial) may be added and more briefly diffused in to help themechanical properties of the material. For example, an initial Ti layerwith a 10-20 mils diffusion interface, where Ta is diffused all the waythrough the interface to form an alloy, and then another Ti layer isadded but diffused less (e.g. so the additional interface is less) toprovide structural support. Thus, the low atomic number material (ordifferent low atomic number materials) may be diffusion bonded twice,once to form an alloy, and the second time primarily for structuralreinforcement.

As further illustrative embodiments, one example material is 145 mil(0.363 cm) thick, with Ti (105 ml/Ta (40 mil). Another material (e.g.for use in a shield or vault or housing) is 125 mil (0.317 cm) thick,with 50 mil Ta (2.15 g/cm²)/35 mil Ti (0.393 g/cm²) that are initiallydiffusion bonded over extended periods of time, and then anotherTitanium layer that is diffusion bonded to the Ti/Ta alloy, that is 40mil (0.461 g/cm²). The additional Ti has greater strength than the Ti inthe extended diffusion bonded layer. The densities of these materialsare Ti-6-4=4.43 g/cm³, Ta=16.68 g/cm³. As yet another example, amaterial (e.g. shield material) is 97 mil (0.248 cm) thick, with 60 milTa (2.58 g/cm²)/37 mil Ti (0.42 g/cm²). This is a Ti/diffusion gradientalloy/Ta Z-grade created by diffusion bonding over extended periods oftime. The densities of these materials are Ti-6-4=4.43 g/cm³, Ta=16.68g/cm³.

As yet another example, a Al/Ti/Ta material may have dimensions of about190 mil (0.483 cm) and an areal density of 3.0 g/cm². In this examples,the Al material is about 89 mils (˜0.226 cm/0.610 g/cm²), the Timaterial is about 63 mils (˜0.160 cm/0.71 g/cm²) and about 40 mil Ta(˜0.102 cm/1.72 g/cm²), for total properties of about 0.488 cmthickness/3.04 g/cm². This example may be made by the Ti/Ta diffusionbonding and then brazing Al to Ti.

For another example material, a Al/Ti/Ta material may have a thicknessof about 240-250 mil at about 3.0 g/cm², with a very thin Ti layer thatacts as interface material for the Al layer (e.g. about 40 mils or less,about 30 mils or less, about 20 mils or less, about 15 mils or less,about 10 mils or less, or about 5 mils or less). Thus, this example isalmost only composed of Al and Ta, with only Ti being used as aninterface. One may ultrasonic weld the Al to Ta directly and this ispreferable to diffusion bonding the Al to Ta, as the Al may melt beforediffusion bonding can occur with Ta, which has a very high meltingpoint. Ti working as an interface, however, can more easily allow thecreation of materials with the desired characteristics. The use of Tifor diffusion and compatibility with the Al brazing is extremelyimportant at bringing together metals that can't go to hightemperatures.

Still other materials may be used for the Z-shielding systems, such anickel-cobalt alloy, or a nickel-cobalt iron alloy (optionally withsmall amounts of other materials such as carbon, silicon, and/or Mn).For example, the commercially available Kovar® allow has been used forsome single layer shielding applications because it can be used forhermetic sealing of spot shields. This material may also braze toaluminum and diffusion bond with Titanium. As illustrated here, anygroup IV metal (or metals) with the necessary thermal properties may beused as the low atomic number material or materials as long as otherdetrimental properties (e.g. poor mechanical characteristics, toxicity,and the like) are present. Other high atomic number materials may alsobe used, such as Group VI metals (with the same caveats noted aboutregarding e.g. mechanical properties), such as Tungsten andTungsten-Copper alloys, or Tantalum alloys such as Ta/W alloys.

To illustrate a vault system and the benefits of the above examplematerials, a vault may enclose an electronic board. In some examples,the board may have outer dimensions of about 9-10 cm×9-10 cm. Using theexample materials above, and assuming board dimensions of 9.0 cm and 9.6cm, and an outer housing dimension of 9.98 cm, the follow calculationsillustrate the benefits of lowering thinness while retaining shieldingcapability.

First, using the 145 mil Z-grade, a 9.98 cm outer dimension minus the0.363 cm for the shielding material thickness gives 9.617 cm.Subtracting the 9.6 cm board dimension effectively leaves no additionalspace, but another surface of shielding material is needed to enclosethe board. Thus, using these example dimensions, it is necessary toobtain an additional 0.346 cm of space from reduction of board dimensionthrough shaving off pieces, removing corners, and the like, rather thanusing the standard size. For the second example material (125 mil),using the 0.319 cm shielding thickness (and using the same calculation),0.256 cm of additional space is needed. For the third example material(97 mil), using the 0.248 cm shielding thickness, only 0.116 cm ofadditional space is needed.

By providing increased density, vaults and enclosures can be made for arelatively small investment compared to the high costs for otherequipment utilized in typical missions (e.g. system electronics, solarpanels, etc.). At the same time, these systems and materials can extendmission lifetimes up to ten years for low earth orbits and eight yearsfor geostationary orbits (compared to typical designed lifespan on theorder of months) with typical electronic cards, allowing the systems toforego more expensive radiation tolerant cards. Thus, the systemsadvantageously allow a reduction in volume while enabling longerduration missions.

These materials, systems and process descriptions are merely examples.In certain embodiments, the materials and systems includes additionalcombinations and/or substitutions of some or all of the componentsdescribed above. Moreover, additional and alternative suitablevariations, forms and components for the materials and systems will berecognized by those skilled in the art given the benefit of thisdisclosure. Finally, any of the features discussed in the exampleembodiments of the processes may be features of embodiments of thematerials and/or systems (or components thereof), and vice versa (e.g.any material examples can be used in any system (such as but not limitedto vaults, housings, enclosures, and spot shields) and any examplematerials described in reference to a system may be utilized as astand-alone material or for other purposes than those discussed in theexample system).

What is claimed is:
 1. A Z-grade alloy material comprising: a highatomic number material; and a low atomic number material, wherein anatomic number of the low atomic number material is lower than an atomicnumber of the high atomic number material, and wherein the low atomicnumber material is bonded to the high atomic number material; andwherein the Z-grade material comprises a diffusion zone, the diffusionzone comprising a mixed metallic alloy material, the alloy materialcomprising both the high atomic number material and the lower atomicnumber material, and wherein the diffusion zone is at least 0.5 mil inthickness.
 2. The Z-grade alloy material of claim 1, The Z-grade alloymaterial of claim 1, wherein the diffusion zone is at least 5 mil inthickness.
 3. The Z-grade alloy material of claim 1, wherein an arealdensity of the Z-grade alloy material is at least about 3.0 g/cm², andwherein an overall thickness of the Z-grade alloy material is about 140mils or less.
 4. The Z-grade alloy material of claim 1, wherein theoverall thickness of the Z-grade alloy material is about 100 mils orless.
 5. The Z-grade alloy material of claim 1, wherein high atomicnumber material comprises one or more of tantalum, tungsten, or acopper-tungsten alloy, and wherein the low atomic number materialcomprises one or more of aluminum or titanium.
 6. The Z-grade alloymaterial of claim 1, wherein the Z-grade material further comprises analuminum layer bonded to the low atomic number material.
 7. The Z-gradealloy material of claim 1, wherein the Z-grade material furthercomprises an aluminum layer bonded to the low atomic number material,wherein an areal density of the Z-grade alloy material is at least about3.0 g/cm², and wherein an overall thickness of the Z-grade alloymaterial is about 190 mils or less.
 8. The Z-grade alloy material ofclaim 1, wherein the diffusion zone is a graded metallic alloy.
 9. TheZ-grade alloy material of claim 8, wherein an additional low atomicnumber material is diffusion bonded to the graded metallic alloy.
 10. AZ-grade vault comprising one or more surfaces of Z-grade material, theone or more surfaces of Z-grade material comprising a high atomic numbermaterial and a low atomic number material, wherein an atomic number ofthe low atomic number material is lower than an atomic number of thehigh atomic number material, and wherein the low atomic number materialis diffusion bonded to the high atomic number material; wherein an arealdensity of the Z-grade material is at least about 2.5 g/cm², and whereinan overall thickness of the Z-grade alloy material is about 240 mils orless.
 11. The Z-grade vault of claim 10, wherein the areal density ofthe one or more surfaces of Z-grade material is at least about 3.0g/cm², and wherein the overall thickness of the one or more surfaces ofZ-grade material is about 100 mils or less.
 12. The Z-grade vault ofclaim 10, wherein the Z-grade material further comprises an aluminumlayer bonded to the low atomic number material, wherein the arealdensity of the Z-grade alloy material is at least about 3.0 g/cm², andwherein an overall thickness of the Z-grade alloy material is about 190mils or less.
 13. A process comprising; combining a high atomic numbermaterial and a low atomic number material, wherein an atomic number ofthe low atomic number material is lower than an atomic number of thehigh atomic number material; and bonding the high atomic number materialand the low atomic number together using diffusion bonding to form aZ-grade material.
 14. The process of claim 14, diffusion bondingcomprises vacuum pressing the high atomic number material and the loweratomic number material at an elevated temperature.
 15. The process ofclaim 14, further comprising vacuum pressing the Z-grade material at anelevated temperature.
 16. The process of claim 15, wherein the elevatedtemperature is near a softening or melting point of the low atomicnumber material.
 17. The process of claim 14, further comprising coolingthe Z-grade material under vacuum.
 18. The process of claim 13, whereinthe diffusion bonding comprises plasma spraying the low atomic numbermaterial onto a sheet of the higher atomic number material.
 19. Theprocess of claim 13, wherein the diffusion bonding comprises welding thelow atomic number material onto a sheet of the higher atomic numbermaterial using an electronic beam gun.
 20. The process of claim 13,wherein the diffusion bonding comprises heating the low atomic numbermaterial under an inert atmosphere or a vacuum to its meltingtemperature, and coating a sheet of the high atomic number material withthe melted low atomic number material.
 21. The process of claim 13,wherein the diffusion bonding comprises ultrasonic layering of the lowatomic number material onto the high atomic number material.